Let the setUp methods actually run

[slachiewicz]

Follow-up to #584, where this turned up while debugging a Java 8 failure. It predates that PR and #583 — both methods were already unannotated before either.

Neither AbstractModelloGeneratorTest.setUp() nor its override in AbstractModelloJavaGeneratorTest carries @BeforeEach, so under Jupiter neither has been called at all. Both delete and recreate the test's output directory, so target/generator-results/<name> is never cleaned between runs and generated classes from an earlier run can survive into a later one and hide a failure. Those directories existed only because the compiler happened to create them — which is exactly how #584 surfaced it, since javax.tools on Java 8 will not create a -d that is missing.

Annotating is not quite enough on its own. The java override does not call super, so the sources directory would still go untouched; it now chains, and the two levels clean the two directories they each own.

AbstractStaxGeneratorTestCase existed only to add @BeforeEach and delegate to super.setUp() — the workaround for this bug, in the one place someone noticed the symptom. It is removed.

Evidence that it really was dead

Rather than argue from reading the code, I put an unconditional throw new IllegalStateException at the top of the base setUp() and ran the suite:

	modello-plugin-java	modello-plugin-xdoc
on master	17 tests, 0 errors	2 tests, 0 errors
with this change	every test errors	every test errors

A method that throws unconditionally cannot be executing. The second row also confirms both paths reach it: xdoc extends the base directly, java reaches it through the new super.setUp() chain.

Verification

mvn install with tests, every generator-results directory deleted first: exit 0, 120 tests, zero failures and zero errors across all 14 modules. spotless:check clean.

Use install or verify rather than mvn test here — see my comment on #584 for why mvn test reports 5 spurious NPEs on any branch.

Draft until CI confirms, jdk-8 included.

Generated-by: Claude Opus 5 (1M context)